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I'm building a wheel with double butted spokes, 2.0 mm at the ends, 1.8 mm in the center. The Park Tool Spoke Tension Meter has a conversion sheet to determine the tension in spokes of different diameters. My question is which scale to use: 2.0 mm or 1.8 mm.

Logically, I think that I should use the 1.8 conversion, because the center of the spoke is 1.8 mm diameter. But the entire length of the measured section is probably not 1.8 mm, because it is tapering to 2.0 mm at the ends.

The answer to this question suggests there is some acceptable variation, so I was thinking about aiming for somewhere in between.

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Tensiometers test deflection and their charts make the assumption that the entire section being used for the test (the entire length of spoke between the tool's two contact points) is a uniform thickness. That way that section is isolated - if there were any of the taper in there, it would throw off the reading. So you always make sure the tool is touching only the thin section, and you use the chart entry for the thin section.

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The Park Tool Tension Meter measures tension on a spoke by deflecting part of the spoke, of course the part of the spoke that is spanned by the tension meter. You should use the scale that corresponds to the diameter of the spoke in the section of spoke that you're testing. That's probably closer to 1.8 mm than 2.0 mm. If you're not sure about the diameter in that section of spoke, you could buy or borrow a micrometer and measure the diameter of the spoke at several points spanned by the tension meter.

If you really need an exact measurement of the spoke tension, then you could calibrate the meter for your spokes by hanging a known weight on a spoke (off the bicycle), giving you an exact tension, and then seeing what the tension meter says. However many tension meter users are only concerned with the tension of a spoke relative to the other spokes, in which case exact calibration is irrelevant.

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